1. Field of the Invention
The present invention relates generally to signal modulators and in particular to spread spectrum modulators for use in clock generators.
2. Description of Related Art
Many electronic devices include some kind of stable clocking circuitry for producing clock signals which allow the devices to operate internally and to co-operate with other devices. The use of highly stable clocks frequently results in electromagnetic interference (EMI). As a result, regulatory agencies such as the Federal Communication Commission (FCC) have established regulations limiting EMI radiation. One approach is to provide shielding and the like, but this approach increases costs and weight. Another approach to reducing EMI emissions is to dynamically vary the clock frequency so as to spread the interference energy over a range of frequencies so as to limit the energy at any one frequency. The approach is known as spread spectrum frequency modulation.
At this point, a brief review of some of the relevant terminology may be helpful. In a typical spread spectrum system, the average system clock frequency (frequency fc) is dithered to some degree, with the peak frequency deviation (Δf) being expressed as free or ±%. The spreading rate (δ) is defined as the range of spreading frequency over the native system clock frequency (Δf/fc). The actual spreading rate δ style can be center-spreading where the frequency deviation Δf is centered around fc (δ=±Δf/2fc×100%); down-spreading where the frequency deviation Δf extends from fc to a lower frequency (δ=−Δf/fc×100%) and up-spreading where the frequency deviation Δf extends from fc to a higher frequency (δ=+Δf/fc×100%). The modulation rate (fm) is the frequency used to determine the system clock frequency spreading-cycling rate. Thus, 1/fm is the period or time during which the clock frequency varies through Δf and returns to the original native frequency. The modulation index (β) is equal to Δf/fm and, finally, the phrase modulation waveform refers to the profile of the clock frequency variation curve as a function of time, with a simple example of a modulation waveform being a saw tooth ramp.
Referring to the drawings, FIG. 1 shows the output frequency spectrum of a non-modulated clock centered at 2 MHz together with the spectrum of a modulated clock centered at near the same frequency but produced using spread spectrum frequency modulation. The modulation waveform is a linear waveform in this case. The center frequency fc for the modulated clock is 2 MHz and the modulation rate fm is 12 kHz. The peak frequency deviation Δf is ±100 kHz or ±5%. The modulation index β is Δf/fm=8.3, and the spreading rate δ is 5. In this example, the spreading style is up-spreading.
As can be seen form FIG. 1, the non-modulated clock produces a relatively high level output concentrated in a narrow frequency range whereas the energy of the modulated clock is spread over a relatively wide frequency band having a much lower peak output. The difference in magnitudes in this case is 11 dB which represents a very substantial reduction in EMI. In this case, the modulation waveform is a linear signal, so that during a given modulation period, the frequency is changed in a linear manner between a value equal to the center frequency plus a fixed percentage and a value equal to the center frequency minus a fixed percentage. In the ideal case, the amplitude of the spread spectrum signal is a somewhat trapezoidal shape having a relatively flat top region indicating that the spectral energy is evenly distributed. In an actual implementation, there will be peaks in the output at various frequencies which tend to reduce the effectiveness of the spread spectrum modulation in reducing EMI.
FIG. 2 show a simplified block diagram of a prior art spread spectrum clock generator. The clock generator includes phase comparator circuit 10 which compares the phase of a reference clock input Fin and a generated clock Fd. The output of the phase comparator 10 is an Up signal and a down signal Dn, with the relative duration of the two signals relating to the phase difference between Fin and Fd. Signals Up and Dn drive a charge pump circuit 12 that sources an output current when signal Up is active and sinks an equal value output current when signal Dn is active. When signals Up and Dn are of equal duration over time, the average output current is zero thereby indicating that inputs Fin and Fd are in phase. A phase difference is indicated by a net current being sourced over time or a net current being sunk over time. The current output CPO of the charge pump circuit 12 is fed to a low pass filter 14 that provides an output voltage Vc relating to the phase difference between Fin and Fd. Control voltage Vc is fed to a spread spectrum modulator 16 which varies the magnitude of Vc is accordance with a spread spectrum modulating profile Mod applied to an input to the modulator.
The output Vc′ of the modulator 16 is applied to the control input of a voltage controlled oscillator (VCO) 18. The frequency of VCO is divided down by an optional divider 20 so that the frequency of Fd matches that of Fin. The modulation waveform Mod applied to the modulator 16 causes Vc′ to vary so that the control signal Vc′ applied to VCO 18 causes Fout to be spread spectrum modulated. Although the FIG. 1 spectrum was produced using a linear modulating profile, other types of profiles can be used. By way of example, FIG. 3 shows the frequency spectrum of an un-modulated clock signal 22 and a spectrum modulated clock signal 24 produced using a sinusoidal modulation waveform. Although there is improved performance on the order of 13 dB over the unmodulated clock, it can be seen that the improvement is limited by the presence of spectrum peaks 26A and 26B. The peaks are produced as a result of the presence of substantial zero slope regions in the sinusoidal modulating profile so that the frequency deviation is momentarily absent thereby resulting in the spikes.
FIG. 4 is an alternative prior art modulation waveform 25 that provides substantially improved performance. It can be seen in this example that the modulating period (1/fm) is about 33 μs, which corresponds to a modulating rate of 30 kHz. In this example, the peak frequency deviation Δf is ±100 kHz, with the center spreading style being used. As described in U.S. Pat. No. 5,631,920, the contents of which are fully incorporated herein by reference, FIG. 5 illustrates a clock generator circuit which is capable of producing and utilizing the FIG. 4 profile using digital circuitry. Among other things, a counter 30 operates to provide a frequency divided output Fin of a reference oscillator 28 to an up/down counter 32. Counter 32 produces addresses used to access a read only memory (ROM) 34 where digital data are stored for producing the FIG. 4 profile. The read data is converted to analog signals by converter 36, with the analog signal on line 44 being provided to an adder circuit 38. A second input to adder circuit 38 on line 46 is produced by phase locked loop circuitry associated with the input clock Fin. The sum of the two analog signals is used to control a VCO 40 which produces the spread spectrum modulated clock Fout. The center frequency of Fout is controlled by the phase locked loop output on line 46, with the frequency deviation be provided by the signal on line 44.
ROM 34 of the FIG. 5 digitally based clock generator along with other circuitry allows the circuit to be programmed to provide substantial flexibility in producing and modifying the FIG. 4 modulation waveform. Although this results in enhanced performance, this performance is achieved at the expense of circuit complexity and power consumption.
There is a need for a spread spectrum clock generator that provides relatively high performance and yet can be implemented utilizing relatively simple analog circuitry while providing reduced power consumption.